Add modsecurity module

[aledbf]

What this PR does / why we need it:

This PR adds the ModSecurity-nginx module to the nginx-slim image.

[k8s-reviewable]

This change is 

[coveralls]

Coverage remained the same at 33.582% when pulling 3f19664f10c7ecb0b3aaaadd3ef09406a63c9be8 on aledbf:nginx-modsecurity3 into c873ef6 on kubernetes:master.

[coveralls]

Coverage decreased (-0.04%) to 33.544% when pulling b44fe5605d4c349f3dcdddded44ba0f74d18fc03 on aledbf:nginx-modsecurity3 into c873ef6 on kubernetes:master.

[coveralls]

Coverage remained the same at 33.582% when pulling b44fe5605d4c349f3dcdddded44ba0f74d18fc03 on aledbf:nginx-modsecurity3 into c873ef6 on kubernetes:master.

[coveralls]

Coverage remained the same at 33.492% when pulling 0447484db4dbcc9d87dc28fce05ea88555c64d95 on aledbf:nginx-modsecurity3 into 63155ee on kubernetes:master.

[coveralls]

Coverage remained the same at 33.492% when pulling a03d6ebbb74efe836f5a52a6456ce9a992c57c65 on aledbf:nginx-modsecurity3 into 63155ee on kubernetes:master.

[coveralls]

Coverage remained the same at 33.492% when pulling 2d23c5120bd7b689a597f0697da160647550510c on aledbf:nginx-modsecurity3 into 63155ee on kubernetes:master.

[coveralls]

Coverage remained the same at 33.492% when pulling a9a22b0 on aledbf:nginx-modsecurity3 into 63155ee on kubernetes:master.

[rikatz]

@aledbf I would only like to ask if that's possible for us to use --add-dynamic-module instead of --add-module, so NGINX creates a .so file, that could be loaded in runtime, allowing us to load this module only if the user want's this :)

Here are some examples, this is supported in NGINX community since version 1.9.11 and may use an absolute path.

[aledbf]

done

[coveralls]

Coverage remained the same at 33.492% when pulling b038e7a on aledbf:nginx-modsecurity3 into 63155ee on kubernetes:master.

[coveralls]

Coverage remained the same at 33.492% when pulling 98e4661 on aledbf:nginx-modsecurity3 into 63155ee on kubernetes:master.

[aledbf]

@nicksardo please merge and publish. Thanks!